Roadside Slope Design for Safety

The design engineer needs objective criteria to aid him in selecting safe combinations of slopes for roadway design and in evaluating alternatives to achieve optimum roadside safety in his design. This paper is concerned with research efforts conducted under this project to provide objective criteria for safe slope combinations and various ditch configurations. The research approach involved simulated traversals of combinations of front and back slope from 3:1 to 6:1 using the TTI version of Calspans Highway Vehicle Object Simulation Model (HVOSM). Curves are presented to aid in evaluation of combinations of slopes forming vee, round, trapezoidal, and rounded trapezoidal ditch configurations having widths to 16 ft (4.9 m). The principal tool used to evaluate or rate the slope combinations at the hinge-point and ditch was the severity index, based on an ellipsoidal mode of failure for tolerance to deceleration forces when these forces are applied to all three axes. The hinge-point region and front slope are also treated. Vehicle rollover does not appear to be a problem for slopes 3:1 or less for standard American-made vehicles.